There are now various portable electrical appliances which can be operated by the user during travelling in other countries. Such appliances may include portable computers, mobile phones and facsimile machines. In order to adapt to the differences in the voltage of municipal power supply, many such products are installed with built-in voltage converter, which allows the user to step up or step down the voltage of municipal power supply, as desired.
Another problem associated with the use of such appliances in various countries is that the shape and arrangement of the holes in wall sockets may vary from country to country. There are now four types of wall-socket arrangements in the world. In Great Britain, Hong Kong, Singapore, etc., the wall sockets are of BS type, and include three rectangular holes. In such countries as the US, Canada, Japan and Taiwan, the wall sockets are of UL type, and include two flat holes. In other European countries, the wall sockets are of VDE type, and include two round holes. In Australia and New Zealand, the wall sockets are of SAA type, and include two inclined flat holes.